Tubular sample heating device for flameless atomic absorption spectrophotometry

ABSTRACT

A hollow electrically conducting tube, typically made of graphite, for holding and atomizing a sample substance for use in flameless atomic absorption spectrophotometry is heated by means of current supplied by electrodes contacting its ends. The improvement comprises providing the inner surface of the tube with grooves extending generally transversely to the longitudinal axis of the tube. This helps to retain sample materials, such as oils, which tend to creep along the inner surface of the tube, especially upon heating. The grooves may be separate and closed upon themselves or in the form of a continuous helix. They may be provided over the entire length of the inside of the tube or over only selected portions, and may be also provided only over the lower half of the tube interior.

United States Patent 1 l ll 3,862,805

Tamm et al. l ll Jan. 28, 1975 TUBULAR SAMPLE HEATING DEVICE FOR FLAMELESS ATOMIC ABSORPTION Primary Examiner-Vincent P. McGraw SPECTROPl-[OTOMETRY Attorney, Agent, or Firm-Salvatore A. Giarratana;

' L. M 1 h K. c t [76] Inventors: Rolf Giinter Tamm, Schlobstr. 5, Franc 16 7777 Salem; Wolfgang Wilhelm Witte, Burgbergring 9, 7770 [57] ABSTRAPT Uberlingen, both of Germany A hollovv electricallyconductmg tube, typically made of graphite, for holding and atomlzmg a sample sub- Filedl J 1974 stance for use in flameless atomic absorption spectro- [21] Appl. No.: 435,337 photometry is heated by means of current supplied by electrodes contacting its ends. The improvement comprises providing the inner surface of the tube with Foreign Application Priority Data grooves extending generally transversely to the longi- Jan. 26, 1973 Germany 7302908[U] tudinal axis of the tube. This helps to retain sample materials, such as oils, which tend to creep along the [52] US. Cl. 356/244, 356/85 inner surface of the tube, especially upon heating. The [51] Int. Cl G0ln 21/16, G01 j'3/30 grooves may be separate and closed upon themselves [58] Field of Search 356/85, 244 or in the form of a continuous helix. They may be provided over the entire length of the inside of the tube [56] References Cited or over only selected portions, and may be also pro- UNITED STATES PATENTS vided only over the lower half of the tube interior. 3,671,129 6/ 1972 Wiedeking 356/85 18 Claims, 7 Drawing Figures PJJENTEU 895 Fig 7 l lg.

Fig 4 Fly 5 Fig. 7

TUBULAR SAMPLE HEATING DEVICE FOR FLAMELESS ATOMIC ABSORPTION SPECTROPHOTOMETRY This invention relates to a tubular, electrically conducting sample accommodating and heating device for flameless atomic absorption spectrophotometry.

Devices of the type indicated are prior art in the form of so-called graphite tubes. Conventional graphite tubes are smooth cylindrical tubes of graphite which are held with conical end surfaces between corresponding annular electrodes. These graphite tubes are commonly provided in the center with a sample injection bore through which a liquid sample can be injected into the graphite tube. Then, a strong electric current is passed through the graphite tube from the electrodes so that the tube is heated to high temperatures. Then, first a drying and possibly ashing of the sample substance is effected. Finally. during a further heating stage the graphite tube is heated up to such a high temperature that the sample is atomized, i.e. within the graphite tube an atomic cloud is formed in which the individual elements contained in the sample are present in an atomic condition. A measuring beam of a spectrophotometer, which beam preferably contains a resonance line of an element of interest contained in the sample, is passed through the hollow graphite tube in the longitudinal direction, so that from the absorption to which the measuring beam is subjected it can be concluded as to the proportion of the element of interest in the sample.

The graphite tube in this usage fulfills several functions. It first serves as a carrier for the sample to be injected. Secondly, by suitable heating of the graphite tube it causes the sample in steps to assume the required evaporating-ashing and atomizing temperatures. Finally, the graphite tube helps to hold the produced atomic cloud together. In order to achieve a quick atomization of the sample and to keep the atomic cloud as long as possible in the interior of the graphite tube, it is desirable that the sample should if possible be kept in the center of the tube. The temperatures to which the tube is heated are at a maximum at the center, since some heat is conducted away at the ends via the electrodes. Moreover, the atomiccloud formed in the center of the graphite tube can be held in the graphite tube the longest before it leaves from the ends of the graphite tube by diffusion and due to the protective gas flow (i.e., an inert gas flow supplied to avoid oxidation of the graphite tube itself at elevated temperatures).

In a prior art graphite tube in the form of a hollow cylinder with smooth inner walls there exists the danger that samples which easily wet the cell surface, such as, for instance, oils, strongly expand upon heating. Such samples can creep as far as the tube ends and can be sucked in by capillary action between the tube ends and the electrodes. In this manner a generally unknown proportion of the sample gets lost before the actual analysis. The (absorption of the) measuring signal is not only attenuated, but also influenced in an unknown, unreproducible manner. The oil in the vicinity of the tube ends, due to the lower temperatures prevailing there, is not decomposed completely thermally, and in the subsequent atomization affects the measurement due to the subsequent formation of further products of decomposition. Moreover, also subsequent measurements (of subsequent samples) can be affected since sample residues in the vicinity of the tube ends cannot be eliminated completely by heating.

It is an object of this invention to so devise a sample accommodating and heating device of the type mentioned hereinbefore that an expansion of the sample in a longitudinal direction-of the tube length is greatly impeded.

It is a further object of this invention to so device a device of the type mentioned hereinbefore that when impeding the expansion of the sample in a longitudinal direction of the tube, the expansion of the sample in a circumferential direction is promoted.

A still further object of this invention consists in so devising a device of the type mentioned hereinbefore that despite the impediment of an expansion of the sample in the longitudinal direction of the tube in dependence on the volume of the injected sample a more or less great area of the inner tube surface can be covered by the injected sample.

According to the invention this object is attained by providing that at least a part of the inner surface of the tube is provided with grooves which extend substantially transversely (i.e., perpendicularly) to the tube axis.

According to this invention grooves extend on the inner side of the graphite tube (or the like) either perpendicularly to the tube axis or at a small angle only with this direction. Then, if a liquid sample is injected in the tube center on the bottom of the horizontally disposed tube, the sample will first fill out one or several adjacent grooves. Creeping of the liquid into the next still empty grooves is impeded by the webs formed between the grooves, particularly if' the webs have sharp edges. If some liquid nonetheless creeps over a web into the next following free groove, it will first accumulate in this groove, since creeping on is again impeded by the following web. Even strongly wetting samples which partly still creep over an adge are thus strongly impeded in their expansion. However, the expansion is not impeded in a peripheral (i.e., circumferential) direction. On the contrary, it is promoted by the capillary action of the grooves. The sample can expand in this direction without leaving the location most favorable for the analysis, that is, the tube center. Thereby, the sample volume which can be accommodated by the tube is increased.

Each groove can be closed on itself; however, the grooves can also follow each other helically.

The entire inner surface of the tube can be provided with grooves. Frequently, however, it is more advantageous if a part or several parts of the inner wall of the tube are kept free of grooves. It is expedient, if the inner wall of the tube is kept free of grooves in the vicinity of the tube ends. Also an area of the inner wall extending axially along the tube: can be kept free of grooves. Then, when used, the tube will be inserted in the apparatus such that this area (free of grooves) is disposed upwardly. Such an arrangement of the grooves with an axial area kept free of grooves can be obtained by forming the grooves with their central axis located eccentrically with respect to the tube axis.

If the tube is provided with a sample injection bore, the inner surface of the tube is expediently kept free of grooves in the area of this sample injection bore. Expediently, the area kept free of grooves extends in the range of the sample injection bore across the total periphery (inner circumference) of the tube. Otherwise,

edged profile. Accordingly, also the cross-section of the webs formed between the grooves can either have a rounded or a sharp-edged profile.

In order -to achieve uniform heating of the tube across its total length, the outer surface of the tube may have annular or helical projections in the range of the internal grooves such that the cross-sectional area of the tube remains substantially constant across the total length thereof.

A few illustrative embodiments of this invention will hereinafter be described more fully with reference to the accompanying drawings in which:

FIGS. 1 to 5 illustrate five different embodiments of this invention in longitudinal section; and

FIGS. 6 and 7 illustrate two different embodiments of this invention in a front view' of the end of the tube.

FIG. 1 illustrates a graphite tube 10 in longitudinal section with helically formed threadedly recessed grooves 12. Between the grooves 12 webs 14 are formed. In the embodiment according to FIG. I the grooves 12 and the webs 14 are of equal width and rounded in their cross-sections. They extend along the total length of the graphite tube 10. The outer wall of the graphite tube is smoothly cylindrical. As in all forms, heating current is supplied by electrodes in contact with each of the ends of the tube 10.

The embodiment according to FIG. 2 differs from the embodiment according to FIG. 1 in three ways. The grooves 16 only extend across a central part of the tube 18. Each groove 16 is closed on itself annularly. The profiles of the grooves 16 and of the webs formed therebetween 20 are triangular. At the ends, the graphite tube 18 has smoothly cylindrical inner walls 22. Reference numeral 24 in FIG. 2 designates a sample injection bore for introducing the sample into the tube prior to heating.

The smooth end parts 22 of the graphite tube 18 prevent break-off of the webs 20 at the ends as could otherwise occur during careless handling or even during manufacture. Moreover, turbulence at the tube ends is reduced when a protective (inert) gas is introduced into the graphite tube 18 through the central sample injection bore 24. Such turbulences could, for instance, direct products of decomposition around the tube ends to the contact bodies (i.e., the electrodes) where they could deposit. During subsequent atomization this deposit could at least partly be freed again and disturb the measurement.

In this FIG. 2 embodiment the smooth end parts have a wall thickness which corresponds to the arithmetic mean value of wall thickness of the grooves and wall thickness of the webs so that no influence on the temperature profile is caused. However, this mean thickness can be deviated from for an intentional influence on the temperature profile, for reasons of manufacture, for reasons of stability (i.e., mechanical strength), for

an increase in the electric contact surface, or the like. By way of example, the wall thickness of the end parts may be made equal to either the wall thickness of the webs 20 or of the grooves 16.

In the embodiment according to FIG. 3, the center of the graphite tube 26 in the vicinity of the sample injection bore 28 is smoothly cylindrical as at 30 and is kept free of grooves. This construction prevents the sample liquid introduced into the graphite tube 26 through the sample injection bore 28 from creeping to the sample injection bore 28. In this embodiment the grooves 32 have a round profile and the webs 34 formed therebetween have a substantially rectangular profile.

FIG. 4 illustrates a graphite tube 36 in which the grooves 38 are substantially wider than the webs 40.

In the embodiment according to FIG. 5 the inner and outer sides of the graphite tube 42 are each provided with a helical thread 44 and 46, respectively such that the wall thickness of the graphite tube 42 remains substantially constantacross the total length thereof.

FIGS. 6 and 7 illustrate two graphite tubes 48, respectively in front view of their ends, The grooves 52 and 54, respectively do not extend across the total periphery, but only cover the lower part of the inner tube surface. In FIG. 6 the grooves 52 are interrupted at half level. In the embodiment according to FIG. 7 the grooves 54 are formed eccentrically relative to the tube axis.

In the above description, tubes or graphite tubes are spoken of without thereby excluding the application of this invention to generally tubular atomizing devices. for instance, of a diameter variable across the length or of a non-circular cross-section, or made of other materials.

What is claimed is:

1. In a hollow tubular, electrically conducting sample accommodating and heating device for flameless atomic absorption spectrophotometry of the type in which heating current is passed through the tube by electrodes contacting its ends for ultimately atomizing the sample substance held within the tube, the improvement comprising:

that at least part of the inner surface of the tube is provided with grooves (12, 16, 32, 38, 44, 52, 54) which extendsubstantially transversely to the longitudinal axis of the tube. 2. A device as claimed in claim 1, in which: each groove (16, 34) is closed on itself. 3. A device as claimed in claim 1, in which: the grooves (12, 44) follow each other helically. 4. A device as claimed in claim 1, in which: the entire inner surface of the tube (10) is provided with grooves (12).

5. A device as claimed in claim 1, in which:

at least part (22, 30) ofthe inner wall of the tube (18,

26) is kept free of grooves.

6. A device as claimed in claim 5, in which:

the inner wall (22) of the tube (18) is kept free of grooves in the vicinity of the tube ends.

7. A device as claimed in claim 5, in which:

an entire part of the inner wallextending axially along the tube is kept free of grooves (FIGS. 6 and 7).

8. A device as claimed in claim 7, in which:

the grooves (54) are'recessed accentrically to the tube axis.

9. A device as claimed in claim 5, in whichi the tube (26) is provided with a sample injection bore (28) and the inner surface (30) of the tube (26) is kept free of grooves (32) in the area of this sample injection bore (28).

10. A device as claimed in claim 9, in which:

the area (30) kept free of grooves in the vicinity of the sample injection bore (28) extends across the entire internal periphery of the tube (26).

11. A device as claimed in claim 1, in which:

the grooves (12) and the webs 14) formed between the grooves (12) are approximately of equal width.

12. A device as claimed in claim 1, in which:

the grooves (38) and the webs (40) formed between the grooves are of substantially different width.'

13. A device as claimed in claim 12, in which:

the grooves (38) are substantially wider than the webs (40).

14. A device as claimed in claim 1, in which:

the groove cross-section has a rounded profile (FIG.

l). 15. A device as claimed in claim 1, in which: the groove cross-section has a sharp-edged profile (FIG. 2 16. A device as claimed in claim 14, in which:

' the cross-section of the webs (14) formed between tially constant across the total length thereof. 

1. In a hollow tubular, electrically conducting sample accommodating and heating device for flameless atomic absorption spectrophotometry of the type in which heating current is passed through the tube by electrodes contacting its ends for ultimately atomizing the sample substance held within the tube, the improvement comprising: that at least part of the inner surface of the tube is provided with grooves (12, 16, 32, 38, 44, 52, 54) which extend substantially transversely to the longitudinal axis of the tube.
 2. A device as claimed in claim 1, in which: each groove (16, 34) is closed on itself.
 3. A device as claimed in claim 1, in which: the grooves (12, 44) follow each other helically.
 4. A device as claimed in claim 1, in which: the entire inner surface of the tube (10) is provided with grooves (12).
 5. A device as claimed in claim 1, in which: at least part (22, 30) of the inner wall of the tube (18, 26) is kept free of grooves.
 6. A device as claimed in claim 5, in which: the inner wall (22) of the tube (18) is kept free of grooves in the vicinity of the tube ends.
 7. A device as claimed in claim 5, in which: an entire part of the inner wall extending axially along the tube is kept free of grooves (FIGS. 6 and 7).
 8. A device as claimed in claim 7, in which: the grooves (54) are recessed accentrically to the tube axis.
 9. A device as claimed in claim 5, in which: the tube (26) is provided with a sample injection bore (28) and the inner surface (30) of the tube (26) is kept free of grooves (32) in the area of this sample injection bore (28).
 10. A device as claimed in claim 9, in which: the area (30) kept free of grooves in the vicinity of the sample injection bore (28) extends across the entire internal periphery of the tube (26).
 11. A device as claimed in claim 1, in which: the grooves (12) and the webs (14) formed between the grooves (12) are approximately of equal width.
 12. A device as claimed in claim 1, in which: the grooves (38) and the webs (40) formed between the grooves are of substantially different width.
 13. A device as claimed in claim 12, in which: the grooves (38) are substantially wider than the webs (40).
 14. A device as claimed in claim 1, in which: the groove cross-section has a rounded profile (FIG. 1).
 15. A device as claimed in claim 1, in which: the groove cross-section has a sharp-edged profile (FIG. 2).
 16. A device as claimed in claim 14, in which: the cross-section of the webs (14) formed bEtween the grooves (12) has a rounded profile (FIG. 1).
 17. A device as claimed in claim 15, in which: the cross-section of the webs (20) formed between the grooves (16) has a sharp-edged profile (FIG. 2).
 18. A device as claimed in claim 1, in which: the outer surface of the tube (42) in the area of the grooves (44) has projections (46) corresponding to the inner surface grooves (44) such that the cross-sectional area of the tube (42) remains substantially constant across the total length thereof. 